One technique for mounting a solid state device, such as an integrated circuit, on a terminal board is known as the "flip-chip" technique. For this technique, the solid state device is provided with small bumps of a tin/lead solder or indium on the terminal pads of the device. The solid state device is then mounted on the terminal board with the solder bumps being seated on corresponding terminal pads on the terminal board. The assembly is then heated to the melting temperature of the solder bumps which then bond to the terminal pads of the solid state device and terminal board. Another technique is to form gold bumps on the terminal pads of the solid state device. The solid state device is then mounted on the terminal board with the gold bumps contacting the corresponding terminal pads on the terminal board. The solid state device is secured in place on the terminal board by a resin.
Although the flip-chip technique provided for ease of mounting a solid state device on a terminal board without the need of connecting terminal wires, for some types of solid state devices it is not practical. For example, solid state image sensors often have features, such as organic color filters, thereon which interfere with the use of the flip-chip technique. Building solder bumps onto such an image sensor is not practical since the topography of the bumps cause uniformity problems for the color filter arrays. Building the bumps onto the image sensor after the color filters have been processed will cause damage to the fragile filters since they cannot withstand the necessary photoresist processes required for making the bumps. In addition, the color filters cannot withstand the high temperatures required to solder the bumps to the terminal board. Therefore, forming the bumps on the terminal board for contact with terminal pads on the image sensor still will not be satisfactory. The resin attachment technique also is not satisfactory since it may effect the image sensing capability of the sensor because of distortions in the light received by the sensor. Also, the usual ultra violet cure for the resin may affect the color filters.
Therefore, it would be desirable to have a flip-chip technique for mounting on a terminal board solid state devices which have features which are adversely affected by high temperatures which can be carried out at low temperatures which do not adversely affect the solid state device. Low temperature solders have been developed for use in bonding parts together, such as the cover plate and housing of an image sensor package, as shown in U.S. Pat. No. 4,895,291 to E. J. Ozimek et al, issued Jan. 23, 1990, entitled "Method of Making a Hermetic Seal in a Solid-State Device" and assigned to the assignee of the present application. However, the problem with such low temperature solders is that if the assembly is subjected to the same low temperatures during the use thereof, the solder will melt and allow the parts to come apart. Therefore, it is desirable to have a bonding technique which not only can be carried out at low temperatures, below the maximum allowable processing temperature, but will withstand much higher temperatures after the bond is formed.